An example of a pressurized fuel channel type nuclear reactor is the CANDU.TM. reactor which contains a plurality of pressure tubes defining fuel channels therethrough. Each fuel channel is horizontally oriented and contains a plurality of fuel bundles, generally arranged end to end. Each fuel bundle contains a set of solid fuel rods or elements containing fissionable material. High pressure heavy water coolant enters the fuel channel at one end, flows over the fuel bundles and through the gaps between the fuel elements so as to cool the fuel elements and remove the heat from the fission process, and exits from the fuel channel at the other end. This heat is subsequently transferred by the coolant to a heat exchanger which produces steam that drives a turbine to produce electrical energy. The heavy water flowing in the water gaps is pressurized and does not boil significantly.
The maximum power that can be produced within a fuel channel is determined by the maximum power that can be produced safely by individual fuel bundles within that channel. This maximum power within the fuel channel is normally referred to as the Critical Channel Power or CCP. The maximum power that can be produced safely by any given fuel bundle within that channel is called the Critical Bundle Power, and is determined by the variation in power production within that bundle, the corresponding local coolant conditions, and the design of the fuel bundle. The Critical Bundle Power is the power corresponding to the onset of a significant decrease in the efficiency of heat transfer from the bundle to the coolant, and the local heat flux at which this happens is referred to as the Critical Heat Flux or CHF. Since the high temperatures that can occur when the CHF is exceeded may damage the fuel bundle, the channel power and flow conditions are set to ensure that CHF is never exceeded in any bundle.
CHF occurs on a heated fuel element when some pan of its surface can no longer be continuously wetted by the liquid coolant. There are two types of CHF, i.e., the departure from nucleate boiling (DNB) type and the liquid depletion type. The actual mechanism for this depends upon the location of the fuel element, and the thermal hydraulic conditions of the coolant surrounding it.
In order to ensure the CHF is never exceeded in any bundle, a safety factor or operating margin is applied to the CCP, which in turn results in the power that can be produced by the pressurized fuel channel type of nuclear reactor being reduced by approximately the same factor. If, however, the CHF could be increased, the power that could be produced by the reactor could also be increased. A similar situation also applies to other types of water-cooled nuclear reactors.
Improvements of CHF on fuel bundles for pressure vessel type light water reactors have been suggested through the following separate methods: (i) addition of extra grid spacers and mixing vanes (U.S. Pat. No. 4,698,204 issued to Taleyarkhan on Oct. 6, 1987); (ii) installation of a tubular flow diverting channel, flow diverting panels or other flow diverting/deflecting devices (U.S. Pat. No. 4,738,819 issued to Taleyarkhan on Apr. 19, 1988, U.S. Pat. No. 4,678,631 issued to Taleyarkhan on Jul. 7, 1987, U.S. Pat. No. 3,663,367 issued to Calvin on May 16, 1972, and Canadian Patent 1,115,863 issued to the Babcock & Wilcox Company on Jan. 5, 1982); (iii) cavitating the element sheath surface (U.S. Pat. No. 4,474,231 issued to Staub et al. on Oct. 2, 1984); and (iv) installation of a special flow distributing channel within the fuel element assembly (U.S. Pat. No. 4,708,846 issued to Patterson et al. on Nov. 24, 1987).
The CHF enhancement methods proposed for pressure vessel type light water reactors are not directly applicable to fuel bundles of pressurized fuel channel type nuclear reactors, such as the CANDU.TM. reactor. This is mainly because of the physical differences between fuel assemblies in the pressure vessel type reactors and the fuel bundles of the pressurized fuel channel type reactors. The pressure vessel type reactor uses very long fuel assemblies which stretch the full length of the reactor, and uses much larger spacing between the individual elements. The type of flow turbulence promoter necessary to provide optimal CHF enhancement in the pressure vessel type reactors, such as extra grid spacers and flow diverting devices, may impose unnecessarily high hydraulic resistance in the fuel channels of the pressurized fuel channel type reactor. Their introduction would also require mechanical changes to the fuel bundle of the pressurized fuel channel type reactor which would affect its overall behaviour. They are therefore not practical for pressurized fuel channel type reactors.
Canadian Patent No. 1,115,863 discloses provision of ridges on guide tubes for control rods. This patent specifies that the subject invention is suitable for preventing early occurrence of the departure from nucleate boiling (DNB) type of CHF, which is likely to occur near the gap between adjacent fuel elements, or between guide tubes and fuel elements. The guide tube in Canadian Patent 1,115,863, is effectively unheated, and therefore has a relatively colder surface than the surrounding fuel elements. The ridges are used to strip liquid off the cold guide tube so that it will be available to the hot fuel elements. The guide tubes are a unique feature of fuel assemblies for pressure-vessel-type, water cooled reactors. In addition, liquid depletion type of CHF is more likely to occur than the DNB type of CHF in a pressurized fuel channel type reactors. The invention described in Canadian Patent 1,115,863 is only applicable to the pressure vessel type of reactor and is not applicable to pressurized fuel channel type reactors where there are no guide tubes.
A method has been proposed, as shown in FIG. 1, to improve CHF in pressurized fuel channel type reactors by the installation of roughness rings 10 on the inner surface of fuel channels 12 (U.S. Pat. No. 3,372,093 issued to Wikhammer et al. on Mar. 5, 1968). The CHF enhancement is accomplished since the roughness rings 10 create turbulence and redistribute liquid from the unheated channel wall to the fuel bundle 14 supported by grid spacers 16 within the fuel channel 12.
This CHF enhancement method proposed for pressurized, fuel-channel type reactors has the following drawbacks:
The presence of roughness ring on the inner surface of the fuel channel creates practical difficulties for the passing of fuel bundle within the fuel channel. The provision of the roughness ring on the inner surface of the fuel channel would therefore require mechanical changes to the fuelling system of the pressurized fuel channel type reactors, which is undesirable. In addition, channel roughness rings increases the hydraulic resistance in the fuel channel significantly. As discussed below, an increase in hydraulic resistance in the fuel channel causes the coolant flow to decrease, and hence causes the CHF to occur at a lower fuel channel power. The resulting CCP is either worse than the case without the CHF enhancement devices, or is only improved to an insignificant degree. High hydraulic resistance may also reduce the coolant flow through the fuel channels in an existing reactor that was not designed to accommodate a large pressure-drop resulting from such a large hydraulic resistance, thus affecting the overall performance of the reactor.